1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a power transmission belt with pulley engaging teeth spaced along the length of the belt.
2. Background Art
It is known to make power transmission belts from rubber with teeth spaced at regular intervals along the length of the belt. These belts are commonly used in high load applications, as in association with engines and other driving/driven devices. It is also known to incorporate and embed in these belts load carrying members, that extend lengthwise of the belt. A cloth layer is commonly placed over a surface of the belt upon which the teeth are formed. Exemplary of such a construction is that shown in Gazette of Japanese Patent Laid-Open 2002/168,302.
In this type of toothed belt, high modulus properties are demanded to maintain engagement of the teeth with a complementarily-shaped and cooperating pulley, even under high load applications. To achieve this objective, it is known to make the cross-sectional area of the load carrying members relatively large. The load carrying members are often made for this high load application by twisting together a plurality of undertwisted cords that together produce the elongate load carrying member. These load carrying members have been made with high modulus fiber, such as glass fiber, carbon fiber, aramid fiber, and PBO fiber.
The use of twisted load carrying members that have a large diameter and that utilize high modulus fiber in a toothed belt has resulted in at least two notable problems. First of all, such belts are prone to having less than satisfactory bending fatigue characteristics. Secondly, these belts tend to experience an initial, potentially detrimental, increase in elongation that results by a tightening of the twisted components of the load carrying members upon running the belt under a high load.
Less than desired bending fatigue properties may cause the following results. Deterioration, resulting from bending increases as the bending repetitions of load carrying members, resulting from engagement with pulleys in operation, increases. This causes elongation of the load carrying members, as a result of which the tension in the belt may be detrimentally lowered. As this occurs, the belt may be prone to premature breakage in a manner by other than separation of the teeth.
As to the second problem, when the initial elongation due to tightening of the twisted components of the load carrying members under load becomes significant, lowering of belt tension in the initial running stages may likewise become significant. This may cause a change in tooth pitch and defects such as tooth cracking caused by uneven abrasion of a cloth layer over the teeth. This low tension condition may make these belts more prone to these defects than a similarly constructed belt operating under a higher, targeted tension.
Still further, a change in tension in the load carrying members due to initial elongation may cause a significant change in their diameters. This may lead to detachment of the load carrying members from the material in which they are embedded.
Both problems are related in a manner whereby attempting to solve one may aggravate the other. For example, by increasing the twisting number of the load carrying members to avoid compromising of the bending fatigue characteristics, the initial elongation may become detrimentally significant. On the other hand, when the twisting number is decreased in order to reduce initial elongation, bending fatigue properties may be worsened.
Heretofore, the industry has not devised a belt construction that adequately enhances durability and belt life by effectively addressing both of the above problems through the selection of thickness, twisting number, etc. of load carrying members.